Plasma processing of semiconductor substrates and other materials, such as for etching or deposition, is typically performed in an evacuated processing chamber in which a plasma is formed in a process feed gas. Microwave radiation may typically be coupled from an external generator, through a radiation transmissive window, such as a quartz or ceramic window, and into the chamber to thereby generate a plasma from the process gas. An ECR apparatus is a typical plasma processing apparatus which operates on the well known principle of electron cyclotron resonance (ECR). Such an ECR apparatus is disclosed, for example, in U.S. Pat. No. 4,877,509 to Ogawa et al.
An ECR typically operates at relatively low pressures, such as between 10.sup.-7 to 10.sup.-2 Torr, and at a relatively low magnetic field strength of 875 Gauss for a microwave frequency of 2.45 GHz to thereby achieve electron cyclotron resonance. In other words, the low pressures mean that the plasmas are in a collision-less regime. Unfortunately, an ECR, because it operates at such low pressures, does not have a sufficiently high processing rate for many desired applications, such as, for example, the deposition of carbon atoms to form diamond semiconductor substrates and layers. Accordingly, the production throughput of such an ECR is limited and increases the cost of production.
In addition, when an ECR is operated at high power in an attempt to increase the processing rate, a relatively hot plasma region is generated adjacent the quartz window. Accordingly, the window then typically requires supplementary cooling, such as by circulating cooling water adjacent the periphery of the window. See, for example, "Plasma Etching with a Microwave Cavity Plasma Disk Source" by Hopwood et al., Journal of Vacuum Science Technology B6(1), January/February, 1988, pp. 268-271. Moreover, contaminants may be introduced into the process gas by etching of the window and adjacent chamber surfaces by the relatively hot plasma region.
A quartz tube positioned within the waveguide coupling power to the plasma has been used to reduce contamination from etching of adjacent chamber surfaces. Such a quartz tube is disclosed, for example, in U.S. Pat. No. 4,866,346 to Gaudreau et al. The use of the quartz tube, however, limits the physical size of the plasma that may be generated and the size of the substrate that may be processed.
Other general microwave and RF driven industrial plasmas are generated in a collision-dominated regime, that is, at higher pressures than ECR's and without magnetic enhancement. However, in high pressure plasma systems the power is absorbed very near the plasma surface. This surface absorption causes much of the plasma energy to diffuse to the walls of the chamber, where it is lost. Moreover, because the energy is absorbed at the surface, the gas temperatures are not uniform over a cross section of the plasma ("skin effects"). Loss of efficiency and non-uniform plasma processing result.
Magnetic fields have not been used with high pressure plasmas, because it has been assumed that a magnetic field will have no effect on a high pressure plasma because the collision frequency is higher than the radiation frequency. The collision frequency increases in a high pressure plasma system because higher pressure means larger numbers of atoms and molecules and more collisions. Accordingly, conventional theory holds that in a high pressure plasma, the large number of collisions will eliminate any effect that could be accomplished with the magnetic field.